pinakes-plugin-api: new UI widget types

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I83a72f3441e5370875239431123b0bbc6a6a6964

crates/pinakes-plugin-api/src/ui_schema.rs

@@ -230,6 +230,37 @@ impl UiPage {
   }
 }
 
+/// A widget that plugins can inject into existing host pages.
+///
+/// Widgets differ from pages in that they are embedded at a specific
+/// `target` location within built-in views rather than occupying a full page.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub struct UiWidget {
+  /// Unique identifier for this widget within the plugin
+  pub id: String,
+
+  /// Target injection point (matches `widget_location` constants)
+  pub target: String,
+
+  /// Content to render at the injection point
+  pub content: UiElement,
+}
+
+/// String constants for widget injection locations.
+///
+/// Use these with `UiWidget::target` in plugin manifests:
+/// ```toml
+/// [[ui.widgets]]
+/// id = "my-widget"
+/// target = "library_header"
+/// ```
+pub mod widget_location {
+  pub const LIBRARY_HEADER: &str = "library_header";
+  pub const LIBRARY_SIDEBAR: &str = "library_sidebar";
+  pub const DETAIL_PANEL: &str = "detail_panel";
+  pub const SEARCH_FILTERS: &str = "search_filters";
+}
+
 /// Core UI element enum - the building block of all plugin UIs
 ///
 /// Elements are categorized into groups: